1. Field of the Invention
The present invention relates to three phase AC motors. More particularly, the present invention relates to a method and apparatus to be used with a motor to detect motor unbalance from changes between voltage and current phases at a test frequency. In a preferred embodiment the test frequency is substantially less than an operating frequency.
2. Description of the Art
Many motors are designed to operate with substantially balanced loads where the load is distributed around the axis of rotation in a substantially symmetrical manner. While such motors can safely drive a slightly unbalanced load, if load unbalance is increased beyond a given point the motor itself and the machine housing the motor could easily be damaged.
Often, a load may appear to be substantially balanced and indeed the load may behave as though it is substantially balanced when operating at a relatively low frequency. However, due to uneven increases in centrifugal force as the operating frequency is increased, often a load that appears balanced at a low frequency will be highly unbalanced at a higher operating frequency. As well known, the centrifugal force F on a load can be expressed as: EQU F=m.omega..sup.2 r (1)
where m is the mass of the load, .omega. is the angular velocity of the motor, and r is the distance from the axis of rotation to the load. When a load is unbalanced, either m1, the mass on one side of a rotational axis, is greater than m2, the mass on the other side or, if m1=m2, r1, the distance of m1 from the axis of rotation, is different than r2, the distance of m2 from the axis of rotation, or both m1 is different than m2 and r1 is different than r2.
Assuming r1=r2 and m1 is greater than m2, referring to Equation 1, as .omega. increase, the centrifugal forces on both sides of the rotational axis increase by a factor of .omega..sup.2. However, because m1 is greater than m2, F1, the centrifugal force due to m1 will increase more than F2, the centrifugal force due to m2. While the ratio F2/F1 might be identical at high and low velocities, the difference between F1 and F2 will be greater at a higher velocity and therefore, a given unbalanced load will be more damaging at a high velocity than at a low velocity.
In some motors where frequency varies by factors of 100, the effects of even a slight unbalance which appear to be negligible at a low frequency, can be devastating at a higher frequency.
One application where motor unbalance is particularly important is in washing machines. Often clothes or other items are thrown into a washing machine in an unbalanced condition where the clothes tend to pile up on one side or the other of a washing basin. When the load is unbalanced, the washing basin wobbles back and forth at high frequencies until, if not corrected, the machine physically moves or a motor component malfunctions. In addition, if the washing basin wobbles enough, the basin can collide with its frame and damage either itself or the frame.
In order to provide washing machines that can operate safely, the industry has utilized mechanical limit switches. A mechanical limit switch is positioned at the outer boundary of what is thought to be tolerable movement of the washing basin. As the rotational frequency of the washing basin is increased, if the load is balanced, the limit switch is never flipped and full operating frequency is achieved. However, if the load is unbalanced, once the rotational frequency of the washing basin is increased sufficiently, the basin will wobble to the point where the limit switch is flipped and motor operation will normally be suspended until a user can manually balance the load.
While limit switches limit motor and washer component damage, for a number of reasons they are unsatisfactory. For example, limit switches allow a considerable amount of rotor and load wobble before they are activated. While the amount of wobble allowed is insignificant in the short term, over a longer period, the effects of even a small degree of wobble can damage motor components. In addition, as the effects of a given unbalance are greater at higher frequencies of rotation due to centrifugal forces, many times a limit switch will only operate once a high operational frequency is attained at which point the effects of wobble can be severe.
Furthermore, limit switches do nothing to try and rebalance a load once unbalance is detected. Once unbalance is detected, a limit switch simply suspends motor operation until a user attempts to rebalance the load.
In other applications, where motor operation is limited to a relatively low frequency, sustained unbalance and resulting wobble can also adversely effect motor operation and eventually result in expedited motor component deterioration.
Thus, it would be advantageous to have an apparatus and/or method that could determine load unbalance at a relatively low motor speed prior to reaching higher frequencies where motor damage is more likely. It would also be advantageous to have an apparatus that could determine even minimal unbalance where a normal operating frequency is a relatively low frequency. In addition, it would be advantageous to have such an apparatus and/or method that could attempt to automatically balance an unbalanced load without user intervention.